AN INVESTIGATION OF TEMPORAL CHANGES IN SOIL PERMEABILITY 
WITHIN THE VADOSE ZONE OF AREAS INUNDATED BY FLOODS

Steven Esling, Bruce DeVantier, Xun Zeng, and Timothy McDonald

Southern Illinois University Carbondale
 

Abstract

Whether or not a particular agricultural chemical reaches an aquifer depends on many factors, including time and rate of application, depth to the first usable groundwater supply, soil organic matter and chemistry, and hydrologic properties of the geologic materials near the surface. The bulk permeability of vadose zone materials is perhaps the most important physical factor controlling the movement of agricultural chemicals beneath the surface. The primary purpose of this study is to investigate how flooding along major rivers may have altered the bulk permeability of the soil. Specific objectives include (1) taking quarterly measurements of the bulk (area average or large-scale) effective permeability of vadose zone materials through the process of soil vapor extraction at three sites recently inundated by floods and three sites that escaped recent floods; (2) identifying the effect that clay mineralogy, particle size distribution, soil moisture content, and density have on effective permeability measurements; (3) comparing point estimates of field-saturated and air permeability determined with field and laboratory permeameters to the effective permeability of air flow determined with soil vapor extraction; and (4) identifying the impact of flooding on the permeability by comparing results obtained for the experimental and control groups. A relatively new technique is used to determine effective permeability in the vadose zone: evaluating pressure drawdown in the soil through time that results from applying a vacuum to a screened borehole. Field test data will be compared to analytical models as well as numerical models developed specifically for the project. 

Seven sites were established on the low terrace and floodplain of the Mississippi River Valley—one of them in an area of controlled flooding, which should effectively guarantee flooding during the period of this investigation. All of the sites target the low-permeability fine-textured overbank deposits with the exception of one site, which has wells installed in coarse-textured deposits. Past work completed included the design and evaluation of the test equipment in an upland site, design and testing of a device for retrieving minimally disturbed soil samples, initial field air permeability and field-saturated hydraulic conductivity measurements from the sites, the collection of soil samples, and laboratory determinations of air and water permeability. Thirteen successful soil vapor extraction tests suggest that macropores control soil air permeability and that macropore distribution is a transient and heterogeneous property of the soil. More recent soil vapor extraction tests have failed. In some cases an elevated water table prevented the test. In other cases, no drop in pressure was observed in the monitoring wells, suggesting a leak somewhere in the system. A grant extension will allow further work this summer to determine what factors lead to a failure of the vapor extraction test in low-permeability materials. This research suggests that the determination of soil permeability by air extraction in fine-grained sediments is feasible, but difficult. The method, however, can reveal temporal changes and spatial differences in soil permeability that other methods miss.